(a) Field of the Invention
The present invention relates to a method and an apparatus for the interferometric wavelength measurement of electromagnetic radiation. The invention is particularly directed toward measuring in a reliable and convenient manner the absolute wavelength of frequency tunable continuous wave laser radiation, using a fringe-counting Michelson-type interferometer.
(b) Description of Prior Art
As is well known, in the Michelson interferometer, an incoming beam of light is split into two beans by a beam splitter. One of the split beams is caused to travel a fixed optical path length by being directed toward a stationary mirror and then reflected back to the beam splitter while the other split beam is caused to travel an optical path length which is varied by being directed toward a moving mirror and reflected back toward the beam splitter; the portion of the interferometer in which one of the split beams travels a fixed optical path length is often called the stationary arm of the interferometer, whereas the portion in which the other split beam travels a varying optical path length is referred to as the variable arm of the interferometer. The reflected beams recombine in a manner depending on the difference between the optical paths they have travelled to provide an interference pattern characterized by a series of alternating bright reinforcement fringes and dark annulment fringes. The bright reinforcement fringes are provided as a result of two waves arriving in phase at a given plane, such as a viewing screen, whereas the dark annulment fringes are provided when two waves arrive out of phase at such plane. These fringes may be easily detected and counted to determine the absolute wavelength of the source of light as the interferometer mirror is moved through a known distance.
The known distance can be accurately obtained by simultaneously counting the number of fringes produced from a standard wavelength source for the same change of optical path length. This principle has been used in the wavemeters proposed by Schawlow et al. in U.S. Pat. No. 4,052,129 and by Hall et al. in U.S. Pat. No. 4,165,183, for accurate wavelength measurement of CW laser light. These devices are based on a two-beam interferometer in which the motion of the interferometer mirror produces the same variation in the respective optical path lengths travelled by a first reference beam of known wavelength and a second beam of unknown wavelength. Thus, by comparing the number of fringes detected for the unknown wavelength .lambda..sub.U with that of the reference wavelength .lambda..sub.R, it is possible to calculate .lambda..sub.U since the ratio of the respective wavelengths is equal to the reciprocal of the ratio of the respective fringe counts.
In the wavemeter of Schawlow et al., the reference and unknown laser beams travel identical paths in opposite directions and emerge at separate detectors. A corner-cube retroreflector coasting on an air track is used as moving mirror. To eliminate systematic errors due primarily to misalignment of the two laser beams with respect to each other, the reference beam and the unknown beam are superposed so as to be colinear, thereby ensuring that the path lengths travelled by the two beams are nearly identical. Practically, this is done when using visible laser beams by visually bringing the laser spots of light into coincidence on at least two optical surfaces of the interferometer which are sufficiently remote from one another. However, when it is desired to align in an invisible laser beam with a visible reference laser beam, the alignment procedure is more complex and necessitates an additional detector for electronically "visualizing" the invisible beam so as to achieve an adequate superposition of the two beams. Thus, such wavemeter is limited in practice and commercial use to the measurement of visible laser wavelengths.
In the device of Hall et al., the beams from the two laser sources travel parallel paths in a double corner-cube interferometer. The beams enter different sectors of the corner-cube retroreflectors which are mounted back-to-back on a moving carriage, and a phase-locked resolution extender incorporating a frequency multiplier provides enhanced fringe resolution. Since the reference and unknown beams are spatially separated, particular care must be taken to avoid any slight misalignment of the beams from parallelism in the direction of translation of the retroreflectors which would result in a systematic error. The alignment procedure proposed by Hall et al is carried out by removing the carriage out of the optical path and adjusting the unknown laser beam so that it coincides with the emerging reference beam. It follows that such alignment procedure is extremely inconvenient especially when the interferometer is evacuated, and renders the wavemeter commercially unattractive.
In summary, the wavemeters of the above type do not permit an automatic alignment of the unknown laser beam with the reference laser beam. It should also be noted that the optical arrangement of both the Schawlow et al. and Hall et al. wavemeters is such that there is no discrimination of the incoming direction of the respective beams, that is, there is always an interference pattern produced regardless of the beam incoming direction.
In order to permit an automatic alignment of the beams parallelism in a wavemeter of the Michelson type, applicant has already shown in his article entitled "Wavelength Comparison Between Lasers," Frontiers in Laser Spectroscopy, North-Holland Publishing Co., 1977, pages 695-712, that a planar mirror should be used in one of the interferometer arms and a corner-cube retroreflector in the other arm. The alignment of the two beams of different wavelengths sent to the corner-cube retroreflector is achieved automatically when the interferometer is adjusted to give a flat interference pattern. In order to observe flat fringes, the two beams of different wavelengths have to be perpendicular to the planar mirror. The provision of a planar mirror in the stationary arm of the interferometer causes a discrimination of the incoming direction of the respective beams so that a flat interference pattern will be produced only at a unique beam incoming direction corresponding to the alignment of the two beams. Since in the system proposed earlier by Applicant, this unique alignment condition is satisfied when the two beams are directed perpendicularly to the planar mirror and thus necessitates using the center of the corner-cube retroreflector, such system suffers from two major drawbacks: optical feedback to the laser source and parasitic diffraction effects by the corner-cube retroreflector. Feedback to the laser is a serious problem as the intensity and the frequency of the laser are perturbed.